Implements StringRef::compare with bounds. It is behaves similarly to strncmp(). Unit tests also included.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129582 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/StringRef.cpp

@@ -29,6 +29,27 @@ static bool ascii_isdigit(char x) {
   return x >= '0' && x <= '9';
 }
 
+/// compare - Compare two strings; the result is -1, 0, or 1 if this string
+/// is lexicographically less than, equal to, or greater than the \arg RHS.
+/// This is different than compare with no size specified as it only 
+/// compares at most the first n bytes.
+int StringRef::compare(StringRef RHS, size_t n) const {
+  // Check the prefix for a mismatch.
+  size_t maxToCmp = min(Length, RHS.Length);
+  maxToCmp = min(maxToCmp, n);
+  if (int Res = memcmp(Data, RHS.Data, maxToCmp))
+    return Res < 0 ? -1 : 1;
+  
+  // Otherwise the prefixes match, so we only need to check the lengths.
+  // Be mindful that if the n is less than or equal to the length of either
+  // string, that is the same as the strings matching because in that case
+  // we only care about the prefix.
+  if (((n <= Length) && (n <= RHS.Length)) || 
+      (Length == RHS.Length))
+    return 0;
+  return Length < RHS.Length ? -1 : 1;
+}
+
 /// compare_lower - Compare strings, ignoring case.
 int StringRef::compare_lower(StringRef RHS) const {
   for (size_t I = 0, E = min(Length, RHS.Length); I != E; ++I) {